Solar collector module

ABSTRACT

Improvements in the design and construction of solar collector modules. Aspects of the invention include improvements in the layout of a frame for a solar collector module, and in the joining of structural members of the frame at nodes. In one example, a module of a solar collector includes a reflector and a three-dimensional structural frame that supports the reflector. The frame is made up of substantially rigid elongate frame members interconnected at nodes. Improved node connection components include hubs or frame members having tabs for receiving frame member connections. A tab may have a width that is substantial in relation to its height. The frame may incorporate asymmetry in its layout, in the size or presence of frame members, or in the positioning of the nodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 13/386,874 filed May 16, 2012 and titled “SolarCollector Module,” which is a national stage entry of PCT Applicationnumber PCT/US2010/043118, filed Jul. 23, 2010, and titled “SolarCollector Module,” which claims priority to provisional U.S. PatentApplication No. 61/228,480, filed Jul. 24, 2009 and titled “SolarCollector Module,” the entire disclosures of which are herebyincorporated herein by reference for all purpose.

BACKGROUND

The trough solar collector is a well-known collector technology used forConcentrating Solar Power (CSP) plants. As shown in FIG. 1, such a planttypically employs a large array of sun-tracking, focusing reflectorsthat concentrate incoming solar radiation onto a tubular conduit thatcontains a working fluid. The focused radiation heats the working fluid,for example an oil or other fluid. The heated working fluid is piped toa central location where its accumulated thermal energy may be utilizedin a conventional heat engine, for example to generate steam that drivesturbines to produce electric power. In other applications, the heatedworking fluid may be used directly, for example where the working fluidis heated water for domestic or commercial use. After its thermal energyhas been utilized, the working fluid may be recirculated through thecollector array to be heated again.

The collector arrays may be quite large, covering several squarekilometers and including thousands of collector modules, such as themodule 101 shown in the simplified diagram of FIG. 1. Several modulesare shown in FIG. 1, each of which has a similar construction. The fieldor array of collectors may be divided into parallel circuits, so thatthe working fluid need not be circulated through the entire collectorfield before it is piped to the central location, but instead may bepassed through a single row of a few dozen modules during each heatingcycle, for example. Many arrangements of circuits are possible. Eachmodule typically includes a parabolic reflector 102 backed by a frame ortruss system 103 on the back side of the reflector (away from the sun).The frame adds rigidity to the module. The modules are typicallysupported on pylons 104 that are located between the modules.

The collector modules are typically grouped into rotatable solarcollector assemblies (SCAs) of several adjacent modules each, connectedin a row. That is, an SCA typically includes several collector modulessupported by pylons in a linear arrangement, such that the collectormodules in each SCA can rotate about a longitudinal axis. For optimumcollection efficiency, all the modules in an SCA preferably rotate inunison to track the sun during the day. Each SCA may be moved by a drivemechanism (not shown) near the center of the SCA, at an end of the SCA,or at another location within the SCA. The collector modules in an SCAmay be coupled to each other using a conventional torque transferassembly that includes a central torsion element (shaft) to coupleadjacent modules. Alternatively, adjacent modules may be coupled neartheir edges or rims, so that torque is transmitted between the modulesprimarily by a force couple acting at the rim and axis of rotation,rather than by torsion of a central shaft. Preferably, the couplingbetween modules accommodates thermal expansion and contraction of theSCA. More description of systems and methods for “edge drive” torquetransfer may be found in co-pending U.S. patent application Ser. No.12/416,536 filed Apr. 1, 2009 and titled “Torque Transfer Between TroughCollector Modules”, the entire disclosure of which is herebyincorporated herein by reference for all purposes.

Torque from at least two different sources is transferred betweenmodules. First, a drive mechanism located near the center of the SCAapplies torque directly to those modules adjacent to the drivemechanism. For the rest of the modules in the SCA, torque is coupledfrom one module to the next so that the entire group of modules in theSCA rotates in unison. Second, the module arrays are also subject towind loading, which may exert very large forces and torques on thearray. Wind loading on each module is transmitted to the adjacentmodule. The resulting torque may be smallest at the end modules of anSCA, but may accumulate through the modules in the SCA row until thedrive mechanism must resist the accumulated torsional wind loading ofmany modules. These torques may be as large as hundreds of thousands ofNewton-meters. In order to maintain proper aiming of the array towardthe sun, the drive mechanism must be able to resist and overcome thetorque resulting from wind loading, and the SCA must be stiff enoughthat no modules deflect enough from optimum aiming that their energycollection performance is degraded significantly. While the torques aregreatest near the drive mechanism, and the modules adjacent the drivemechanism must resist the largest torques, the deflection may accumulateoutward from the drive mechanism, and may be greatest at the end of theSCA furthest from the drive mechanism.

In order to achieve enough stiffness, the frame or truss system 103should be designed to withstand the expected torques with acceptablysmall deflection. Also, the coupling of two or more optically-precisedevices, such as the modules of an SCA, requires that the assembly befabricated with a relatively high degree of precision for proper energycollection. In addition, it is desirable that each module be light inweight, easy to assemble, and low in cost. In large part, thesecompeting design goals—stiffness, accuracy, light weight, ease ofassembly, and low cost—are dependent on the design of the frame or trussportion of the collector modules. There is accordingly a need forimproved frame designs for use in solar collector modules.

SUMMARY

Embodiments of the invention relate to improvements in the design andconstruction of concentrating solar collector modules. Aspects of theinvention include improvements in the layout of a frame for a solarcollector module, and in the joining of structural members of the frameat nodes.

According to some embodiments, a hub is configured to join frame membersin a space frame truss. the hub includes a main portion and at least onetab protruding from the main portion, the tab having two spaced apartouter sides and a spanning surface joining the outer sides. A tab widthextends across the outer sides at that main portion and a tab heightextends from the spanning surface to the main portion, and the tab hasan aspect ratio that is the ratio of the width of the tab to the heightof the tab, and the aspect ratio is between 0.25 and 4.0.

According to other embodiments, a connection at a node of a space frametruss comprises a generally tubular elongate frame member having aconstant cross sectional shape along its length. The frame membercomprises two generally flat sides defining a portion of the crosssectional shape of the frame member and at least one joining side thatextends between the two generally flat sides for closing the tubularframe member cross sectional shape. A portion of the frame memberjoining side at an end of the frame member is removed such that the twogenerally flat sides protrude beyond the remainder of the frame member.The connection further comprises a hub having a main portion and aprotruding tab, the tab having two spaced apart outer sides and a topsurface joining the outer sides, and the two flat sides of the framemember and the two outer sides of the tab are cooperatively sized suchthat the two protruding flat sides of the frame member fit over the tab.

According to other embodiments, a connection at a node of a space frametruss comprises a hub having a main portion and a protruding tab, thetab having two spaced apart outer sides and a spanning surface joiningthe outer sides. The connection also includes a tubular frame member andat least one connector fixed to the frame member to form a frame memberassembly. The connector protrudes from an end of the frame member andincludes at least one feature for assembling the connector to the tab.

According to other embodiments, a solar collector module comprises areflector configured to direct incoming solar radiation onto a receiver,and a three-dimensional structural frame to which the reflector ismounted. The frame includes a plurality of interconnected substantiallyrigid elongate frame members, and frame has two ends. The frame alsoincludes at each end a fitting displaced from an axis of rotation of themodule, each fitting configured to participate in the transfer of torquebetween the module and an adjacent module via a direct connectionbetween the modules at a location displaced from the axis of rotation.The three-dimensional structural frame is asymmetrical about a centrallongitudinal plane.

According to other embodiments, a solar collector module, comprises areflector configured to direct incoming solar radiation onto a receiverand a three-dimensional structural frame to which the reflector ismounted. The frame includes a plurality of interconnected substantiallyrigid elongate frame members connected at nodes. The three-dimensionalstructural frame comprises an upper surface defined by a set of uppernodes and a lower surface defined by a set of lower nodes, with eachlower node being connected to at least one upper node by a strut. Thesolar collector module also includes a monolithic hub at each node towhich each frame member reaching the respective node is connected, andno frame member reaches more than two nodes.

According to other embodiments, a three-dimensional structural framecomprises a set of upper nodes defining an upper surface, the uppernodes arranged in generally longitudinal rows, and a set of lower nodesdefining a lower surface, the lower nodes arranged in generallylongitudinal rows. The frame also includes a plurality of substantiallyrigid elongate struts, and each node in the upper surface is connectedto at least one node in the lower surface by at least one of theplurality of struts. The frame further includes at least onesubstantially rigid elongate chord member extending to at least twonodes in a particular row of nodes, the chord member having asubstantially constant cross sectional shape throughout its length. Thechord member further comprises at least one strut connection featuresuch that any strut connecting to any node in the particular row ofnodes connects to the chord member.

According to other embodiments, a three-dimensional structural framecomprises a set of upper nodes defining an upper surface, the uppernodes arranged in generally longitudinal rows and a set of lower nodesdefining a lower surface, the lower nodes arranged in generallylongitudinal rows. The frame further includes a plurality ofsubstantially rigid elongate struts, wherein each node in the uppersurface is connected to at least one node in the lower surface by atleast one of the plurality of struts, and at least one substantiallyrigid elongate chord member extending to at least two nodes in aparticular row of nodes, the chord member having a substantiallyconstant cross sectional shape throughout its length. The frame alsoincludes, at a node reached by the chord member, at least one channelhaving a throat and two spaced apart sides. The channel is affixed atthe throat to an outside surface of the chord member, and each channelis configured to receive at least one strut reaching the respectivenode, to connect the at least one strut to the respective node.

According to other embodiments, a solar collector module comprises areflector configured to direct incoming solar radiation onto a receiver,and a three-dimensional structural frame to which the reflector ismounted. The frame comprises a plurality of interconnected substantiallyrigid elongate frame members, wherein the frame members are connected atnodes. The frame comprises an upper surface defined by a set of uppernodes and a lower surface defined by a set of lower nodes, each lowernode connected to at least one upper node by a strut, and the nodes arearranged in generally longitudinal rows. The solar collector modulefurther includes at least one substantially rigid elongate chord memberin each row of the upper surface, connecting least two nodes in therespective row, and a plurality of discrete standoffs affixed to thechord members in the top surface, the standoffs at least partiallysupporting the reflector.

Other features and advantages of the present invention should beapparent from the following description of the preferred embodiments,which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic view of a portion of a concentrating solarpower plant.

FIGS. 2A-2D show oblique, bottom, end, and side views respectively of asolar collector module, in accordance with example embodiments of theinvention.

FIGS. 3A-3G depict hubs for connecting members of a frame at nodes, inaccordance with embodiments of the invention.

FIG. 4 depicts a connection of a frame member to a hub, in accordancewith embodiments of the invention.

FIGS. 5A-5L illustrate connectors, according to embodiments of theinvention, for connecting frame members to hubs.

FIGS. 6A and 6B illustrate schematic end and oblique views of a genericcollector module.

FIGS. 7A-7H depict frames for solar collector modules, in accordancewith embodiments of the invention.

FIG. 8A shows a portion of a frame for a collector module, in accordancewith other embodiments, and

FIG. 8B shows an overall view of the frame of FIG. 8A.

FIGS. 9A and 9B depict portions of a frame for a collector module, inaccordance with other embodiments of the invention.

FIG. 10 shows a means of connecting members in a solar collector moduleframe, in accordance with other embodiments of the invention.

FIGS. 11A and 11B show portions of solar collector modules, inaccordance with other embodiments of the invention.

DETAILED DESCRIPTION

In general, embodiments of the invention relate to improvements in thedesign of a truss or frame for a solar collector module.

FIGS. 2A, 2B, 2C, and 2D show lower oblique, bottom, end, and side viewsrespectively of a solar collector module 200, in accordance with anexample embodiment of the invention. Certain parts of the module 200 areomitted for clarity of explanation, but the module 200 will serve toillustrate the overall structure of a module, and some terms used todescribe portions of a module.

Referring to FIGS. 2A-2D, the solar collector module 200 includes areflector 201 and a frame 202. The reflector 201 may be, for example, inthe shape of a parabolic cylinder or other curved shape configured toreceive incoming solar radiation 203 and concentrate it onto a linearreceiver tube 204 that carries the working fluid. The reflector 201 maybe made, for example, of a single piece of reflective material, forexample plated or polished sheet steel or aluminum, glass mirrors, oranother highly reflective material, or may be made of multiple pieces.In a preferred embodiment, the reflector is made of multiple curvedglass mirror segments that cooperate to define the curved shape of thereflector 201. In FIG. 2C, the reflector 201 is omitted for purposes ofillustration of the module components.

While embodiments are described in relation to a curved reflector thatconcentrates solar radiation onto a receiving tube, it will berecognized that other embodiments may utilize other reflector shapes,for example a flat reflector that is part of a heliostat directing solarradiation onto a remote receiver.

The module 200 has a length, measured in a longitudinal directionparallel to an axis of the curved cylinder defined by the reflector 201.The module 200 has a width, measured transverse to the length andbetween edges 205 and 206. In a typical power generation application,each module may be about 8 or 12 meters in length, and about 3.5 or 5meters in width, although other sizes and aspect ratios are possible.

The reflector 201 is mounted to the frame 202, which imparts thestiffness and strength to the module to maintain the proper shape andorientation of the reflector 201. The frame 202 may be thought of as aspace frame truss, composed of multiple members joined at nodes of thetruss. There may be at least four different kinds of truss members:chords, ribs, diagonals, and struts. In addition, other kinds of membersmay be present, for example purlins and torque arms.

As is most easily seen in FIGS. 2A and 2B, chords or chord members runin the longitudinal dimension of the frame 202, and lie in the surfacesof the space frame truss making up the frame 202. In the example module200, there are three upper chord locations, including two upper rimchord locations in which chord members 207 and 208 (and other chordmember) reside, and a center chord location, in which chord member 209resides (along with other chord members). There are also two bottomchord locations, in which chord members 210 and 211 (and other chordmembers) reside. Other numbers of chords are possible. In someembodiments, each upper chord is a single chord member that runs thelength of the frame 202, and the lower chords are somewhat shorter thanthe length of the frame 202. As will be explained in more detail later,in other embodiments, multiple chord members are placed along a chordlocation.

Ribs run generally transverse to the chords, and are connected betweenadjacent chords. For example, upper ribs such as ribs 212 and 213 lie inthe upper surface of the frame 202, each connecting one of the rimchords to the upper center chord 209. Bottom ribs such as ribs 214 and215 lie in the bottom surface of the frame 202 and connect the bottomchords 210 and 211.

Diagonals also lie in the frame surfaces, but are connected diagonallybetween non-aligned nodes. Examples are illustrated by upper diagonals216 and 217, and lower diagonals 218 and 219.

Struts generally connect between the two surfaces of the frame 202. Forexample, a strut may connect to a node in the upper surface and a nodein the lower surface of the frame 202. (In an occasional exception,lower diagonals may sometimes be referred to as bottom struts.) As mosteasily seen in FIG. 2C, outer struts, exemplified by struts 220 and 221connect between nodes along the rim chords 207 and 208 and the bottomchords 210 and 211. Inner struts, exemplified by struts 222 and 223,connect between the center chord 209 and the bottom chords 210 and 211.

The locations in the frame 202 where frame members meet are referred toas nodes. In some embodiments, a fitting is present at each node tofacilitate the connections of the various frame members meeting at therespective node. This fitting may be referred to as a hub or nodeconnector, and may be a unitary piece or made up of multiple pieces.

In addition, purlins, exemplified by purlins 224 and 225, may runlengthwise on the top surface of the frame 203, and may provide mountingsurfaces for the segments of the reflector 201. Finally, torque arms 226and 227 may be provided at the ends of the module 200, for assisting inholding the receiver tube 204 in the proper location and moving thereceiver tube 204 as the module 200 rotates.

While the example module 200 serves to illustrate some of theterminology used to describe module features, it should be understoodthat not every embodiment will include all of the features shown in themodule 200, and other embodiments may include similar features insomewhat different configurations than shown in FIGS. 2A-2D.

Hubs

FIG. 3A depicts a hub 301 in accordance with an example embodiment ofthe invention. The hub 301 is particularly suited for connecting struts,ribs, and diagonals to an upper rim chord, but the principlesillustrated by the hub 301 may be embodied in hubs for use in otherlocations in a frame such as the frame 202. The example hub 301 includesa main body portion 302 having a longitudinal axis 303. The main bodyportion 302 encloses an open passage 304 through which a chord or otherframe member may pass. The passage 304 may have a size andcross-sectional shape that match the profile of the chord, which mayhave any suitable cross sectional shape, including round, square,oblong, generally round with flattened sides, or another shape. The hub301 has integral box-shaped tabs 305, configured to be captured by framemembers or frame member end pieces. Other prior hubs capture framemembers between opposed parallel fins, or utilize single, thin fins thathave limited strength. A single fin 320 is illustrated in FIG. 3A forpurposes of illustration. The hub 301 may be manufactured using anextrusion process. The configuration of the hub 301 with the box-shapedtab 305 minimizes the size of the hub cross-section (an importantelement of the extrusion process), allows utilization of larger framemembers that need not fit between fins, provides pairs of stablesurfaces to which conjoining frame members may be fastened, and makesthe tab elements stronger and more dimensionally-stable duringfabrication and field operation.

Each of the tabs 305 comprises spaced-apart outer sides 306 and aspanning surface 307 joining the outer sides 306. (The transitionsbetween the sides and top surface may be rounded, chamfered, orotherwise shaped to ease the transition over a small distance, but thespanning surface 307 is still considered to join the sides 306.) Eachtab 305 has a width W and a height H, and an aspect ratio W/H. Not alltabs on a hub need have the same dimensions. For the purposes of thisdisclosure, the width of a tab is measured at the root of the tab,adjacent the main portion 302 of the hub 301, between the sides 306 andthe height of the tab is measured from the outer surface of the mainportion 302 at the root of the tab to the spanning surface 307. Theaspect ratio W/H of each tab is preferably between 0.25 and 4.0, andmore preferably between 0.33 and 3.0, and even more preferably between0.5 and 2.0. In any case, each tab is wider than a thin fin, and has awidth that is substantial in relation to its height. By contrast, atraditional fin has a width that is much smaller than its height. Thesize and orientation of the tabs 305 can be varied depending on the sizeof the member(s) or terminations(s), and on the desired frame geometry.In the example hub 301, the tabs 305 are hollow. For added strength, thetabs may be internally reinforced, or even made solid. The hub 301 maybe referred to as a “tabbed” hub. The lengths of a particular hub andits tabs may be selected as necessary to accommodate the number of framemembers meeting at the particular hub.

In some embodiments, each of the tabs 305 is positioned so that thelines of action of the frame members attached to a particular tab passthrough a common point within the hub, so that joint eccentricity isavoided. This may be accomplished in part, for example, by having eachtab be symmetrical about a respective plane of symmetry 308, andarranging for each of the planes of symmetry to include the longitudinalaxis 303 of the main portion.

The hub 301 is conveniently made of extruded aluminum, although othermaterials and processes may be used. For example, the hub 301 may becast, welded together, or otherwise formed from aluminum, steel, apolymer, a composite, or another suitable material. In some embodiments,the tabs 305 may be attached using fasteners or other means, for examplerivets, screws, bolts, adhesive bonding, welding, or an integralattachment feature such as a dovetail joint.

FIG. 3B illustrates the hub 301 in use to connect an upper rib 212, anupper diagonal 216, and a strut 220 to the rim chord 207. The variousframe members may be attached to the hub 301 using pins, bolts, rivets,screws, or other suitable fasteners. Tabs 305 may include holes forreceiving fasteners. FIGS. 3C and 3D illustrate two other hubconfigurations 309 and 310, particularly useful along a bottom chord andan upper center chord respectively. In addition, FIG. 3D illustratessome other tab configurations in accordance with embodiments, includingan internally reinforced tab 321 and a solid tab 322. Tab 323 includesprotrusions 324 that protrude beyond spanning surface 325, which joinsthe outer sides of the tab 323. The tab 323 is still considered to be abox-shaped tab within the scope of the appended claims. In the examplesshown thus far the side walls of the tabs are planar and parallel, thetop surface is planar, and the box-shaped tabs are generally rectangularin cross section. Other shapes are possible, including box shapes withnon-parallel sides, sides of different lengths, non-constant crosssectional shapes, or other suitable shapes. For example, FIG. 3Eillustrates a hub 311 having a tab 305 with non-parallel sides 312 and313.

FIG. 3F illustrates another example hub 314 having tabs. The hub 314includes a main portion 315 having a longitudinal axis 316. Tabs 317 and318 protrude from the main portion and extend in a direction parallel tothe longitudinal axis. The hub 314 does not include a passage configuredto receive a frame member through the passage. Each tab may have a widthand a height and an aspect ratio that is the ratio of the width to theheight. The aspect ratio is preferably between 0.25 and 4.0, and morepreferably between 0.33 and 3.0, and even more preferably between 0.5and 2.0. FIG. 3G illustrates yet another example hub 319 having tabs.Conveniently, a hub may include mounting holes such as mounting holes326 shown in the hub 319, for attaching torque arms and the like.

While the exemplary hubs shown in FIGS. 3A-3G are conveniently made byextrusion and therefore have longitudinal axes, this is not arequirement. For example, hubs according to embodiments of the inventioncould be made by casting or other processes, and may not necessarilyhave shapes formable by extrusion. For example, a hub designed to beused in a node similar to the node shown in FIG. 3F could have agenerally hemispherical base shape with a tab protruding in thedirection of each frame member to be attached to the hub. Many otherconfigurations and manufacturing processes may be utilized.

Frame Member End Designs

In some prior designs, all of the frame members in a given plane were ofthe same cross sectional size and shape, which necessarily were selectedbased on the requirements of the frame member expected to carry thehighest load. As such, other frame members were overdesigned, and thecost, weight, and material requirements for such a frame wereunnecessarily high. this limitation also constrained the range of framemember sizes available to the designer to those which could interfacewith a realistically manufacturable hub, thereby limiting the members'ability to carry axial compression loads over long distances andimposing limits on the geometric layout of the overall frame.

Preferably, each frame member in a collector module frame embodying theinvention is sized appropriately for the load it is expected to carry.In some embodiments, the frame members are generally tubular. Anyparticular frame member may be round, oval, rectangular, square,generally round with flattened sides, or of another cross sectionalshape. The frame members need not all be of the same shape or size. Forexample, one frame member may have a different wall thickness thananother frame member, or may have a significantly different diameterthan another frame member, or may differ in other ways. Because theframe members may vary in size and shape, various ways of connecting theframe members to the hubs may be used within a single module.

FIG. 4 illustrates one example technique for attaching a frame member401 to a hub 402. The frame member 401 may be, for example, a strut,rib, or diagonal. The example frame member 401 is generally tubular, andhas a constant cross sectional shape along its length, as convenientlyresults if the frame member 401 is extruded. The frame member 401 hastwo generally flat sides 403 and 404. (Part of each of generally flatsides 403 and 404 has been thickened for reinforcement, but the twosides are still considered to be generally flat.) The remainder of thecross section, such as joining side 405, may have any suitable shape,for example curved or straight portions making up the remainder of theperiphery of the tubular member. The example hub 402 is similar to thehubs described above, and comprises a main portion 406 that has alongitudinal axis 407, indicated by a dashed line. One of skill in theart will recognize that the connection technique shown in FIG. 4 couldbe used with other kinds of hubs as well, for example hubs that includea passage through which a frame can be received. The hub 402 alsocomprises a tab 408 that has two spaced apart outer sides 409 and 410,and a spanning surface 411 that joins the outer sides 409 and 410. Thegenerally flat sides 403 and 404 of the frame member are cooperativelysized with the two outer sides 409 and 410 of the tab 408 such that thetwo generally flat sides 403 and 404 of the frame member 401 canstraddle the tab 408, preferably in a sliding, loose sliding, orclearance fit. For example, a tab may have a nominal width of between 30and 100 millimeters, and generally flat sides 403 and 404 of the framemember may be nominally spaced apart by a distance about 0.1 to 3.0millimeters greater than the width of the tab to which the frame memberis to be connected.

Other parts of the frame member 401 are removed so that the generallyflat sides 403 and 404 protrude beyond the remainder of the framemember. Tab 408 may have an aspect ratio (its width divided by itsheight) of between 0.25 and 4.0. One or more fasteners such as a pins,rivets, bolts, screws, or other fasteners may be used to join the framemember 401 to the hub 402. A single fastener may be used in a joint thatneed only react to axial loads in the frame members. Multiple fastenersmay be used to create a joint that can also resist moments. FIG. 4illustrates a joint with multiple holes 412 through the generally flatsides 403 and 404 of the frame member 401, and through the tab 408, forreceiving multiple fasteners (not shown).

The kind of connection shown in FIG. 4 makes a connection between theframe member 401 and the hub 402 using a minimum number of parts. Notransition members or other connectors are needed. This kind of jointmay be especially useful for frame members whose expected loads allowthem to be of a size comparable to a suitable tab size on a particularhub. In some cases, it may be possible to specify the wall thickness orother dimensions of the frame member 401 such that the frame member 401has the appropriate strength and stiffness and also a size compatiblewith the hub 402. The tab 408 and the frame member 401 may be designedcooperatively to enable use of this kind of connection.

In another embodiment, transitional pieces called “connectors” may beprovided between frame members and hubs. A connector is an additionalstructural element fixed to the frame member, and having features suchas holes that facilitate the connection of the frame member to a hub.The combination of the frame member and the connector may be called aframe member assembly.

The connectors may provide various benefits, including easing thetransition between the frame member shape and the hub shape, enablingthe use of larger frame members, providing additional strength, or otheradvantages. A wide variety of connector styles is possible. Any workablecombination of connector and frame member geometry may be used. Forexample, frame members maybe round, square, rectangular, generally roundwith flat sides, or any other suitable shape. Not all of the members ina frame need have the same shape, and not all connections within a frameneed use the same connector style.

FIG. 5A illustrates a connection in accordance with one embodiment. Inthe example connection of FIG. 5A, a hub 501 has a longitudinal axis 502and comprises a protruding tab 503. (When referring to a longitudinalaxis of a hub, the axis is a longitudinal axis of the hub itself, and isnot necessarily aligned with any longitudinal axis of a frame in whichthe hub resides.) The tab 503 extends in a direction parallel to thelongitudinal axis 502 and has two spaced apart outer sides 504 and 505,and also has a spanning surface 506 joining the two sides 504 and 505. Atubular frame member 507 includes two connectors 508 fixed to the framemember 507 to form a frame member assembly. The connectors 508 are flatplates, affixed to the inside of the tubular frame member 507. Theconnectors 508 may be, for example, formed from sheet steel or aluminumor another suitable material, by stamping or another suitable process.Each of the connectors 508 includes at least one feature for assemblingthe connector 508 to the tab 503. In this example, the features areholes through the connectors cooperatively positioned with correspondingholes in the tab 503 to receive a fastener such as a pin, bolt, rivet,screw, or another kind of suitable fastener. Either or both of theconnectors 508 could be fixed to the outside of the frame member 507,rather than the inside. The connectors 508 may be fixed to the framemember 507 by any suitable fastening scheme, including welding or theuse of fasteners such as rivets, bolts, or screws. The connectors 508protrude from an end 509 of the frame member 507. One or more connectorsmay also be provided at the opposite end of the frame member 507. Thetwo connectors 508 are spaced apart to fit over the tab 503, preferablyin a loose sliding fit, straddling the sides 504, 505.

FIG. 5B illustrates a connection in accordance with another exampleembodiment. In this example, connectors 510 comprise enclosed hollowshapes. While the shapes shown in FIG. 5B are rectangular, one of skillin the art will recognize that other shapes may be used. In the exampleshown, the ends of the connectors 510 have been additionally formed toprovide clearance for connections to other tabs on the hub.

FIGS. 5C and 5D illustrate connections in accordance with other exampleembodiments. In these examples, the connectors 511 are bent plates. Thiskind of connector may be especially suited to connecting large framemembers to hubs. These embodiments also illustrate that the connectors511 may be mounted to the inside or outside of the frame member.

FIG. 5E illustrates connections in accordance with two otherembodiments. The connections shown in FIG. 5E include a hub 512 aspreviously described, having a main portion 513 and a tab 514. Twotubular members 515 and 516 are connected to the tab 514. The connectionof the tubular member 515 uses two connectors 517 having open channelshapes fixed to the inside of the tubular frame member 515. In thisexample, the frame member 515 is rectangular. The connection of thesecond tubular frame member 516 uses two connectors 518 stamped incompound shapes. That is, the connectors 518 have curvature in twodirections. This kind of connector may be especially useful forconnection round or near-round frame members such as the frame member516 to hubs.

FIG. 5E also illustrates connections using single or multiple fasteners.The connection of the frame member 515 uses only a single fastener (notshown) such as a pin, bolt, rivet, or screw through hole 519 andcooperating holes in the tab 514. The connection of the frame member 516uses two fasteners (not shown) through holes 520 and cooperating holesin the tab 514. This kind of connection is able to withstand moments, aswell as axial loads in the framing member 516.

FIG. 5F illustrates a connector 521 in accordance with anotherembodiment. The connector 521 may be conveniently formed by stampingfrom a single piece of sheet material such as steel or aluminum.Alternating straps 522 are formed by distending alternating portions ofa flat blank toward opposite sides of the blank. The resulting straps522 are configured to form a receptacle to receive an end of a framingmember (not shown). The connector 521 may be fixed to the framing memberby welding, rivets, screws, bolts, or any other suitable attachmentscheme. The connector 521 includes a flat end portion 523 havingfeatures for assembling the connector 521 to a hub (not shown). In thisexample, the connection features are holes 524 suitable for receivingpins, bolts, rivets, screws, or other fasteners in cooperation withholes in a tab of the hub. Other variations of the connector 521 arealso possible. For example, the straps 522 need not be orientedperpendicular to the longitudinal axis of the connector 521, but couldbe oriented at some other angle. Advantages of such “angled” stampinginclude ease of assembly and enhanced joint strength.

FIG. 5G illustrates a connector 525 in accordance with otherembodiments. The connector 525 is preferably made from an extruded shapehaving a cross section sized and shaped to fit within a frame member,and modified to fit with the desired hub. The modifications may includenotching portions of the extruded shape, and drilling holes for mountingto frame members and hubs. A connector such as the connector 525 caneffectively join a large diameter frame member to a relatively small tabon a hub, using a single part. The connector 525 may be fabricated tofit any of a variety of frame member shapes.

FIG. 5H illustrates the connector 525 mounted in a frame member 526 andpositioned to attach to a hub 527. Any suitable fasteners may be used toconnect the connector 525 to the frame member 526, for example rivets,screws, bolts or pins through holes 528 of the frame member 526 andholes 529 of the connector 525. The connector 525 could also be weldedto the frame member 526. Similarly, rivets, screws, bolts, pins or othersuitable fasteners may be used to connect the connector 525 to the hub527, for example via holes 530 in the connector 525 and mating holes inthe hub 527. Portions 531 of walls of the connector 525 may be thickenedin relation to other walls, to increase the bearing capacity of theconnector 525 and reduce the likelihood of material failure at the holes530 when the connector is under load. The connector 525 may beconfigured to fit within a frame member having a round interior opening,and thus may be useful for making connections in frames that utilizesteel frame members, which may be available in only a limited number ofcross sectional shapes.

FIG. 5I illustrates a connector 532 in accordance with otherembodiments. Connectors such as the connector 532 may be used in mirrorimage pairs to form a connector similar to the connector 525 describedabove. FIG. 5J illustrates a connection of a frame member 533 to a hub534, using a mirror image pair of the connectors 532. The connectionshown in FIG. 5J provides many of the advantages provided by theconnection shown in FIG. 5H. The connector 532 may also be formed byextrusion, with subsequent notching and drilling, but may also be formedby other methods, for example stamping or bending of sheet metal.

FIG. 5K illustrates a connector 535 in accordance with still otherembodiments. The connector 535 may also be formed principally byextrusion and cut to the appropriate length. Holes for mounting theconnector 535 to frame members and hubs may be added by drilling orother methods. FIG. 5L illustrates the connector 535 as used to make aconnection between a frame member 536 and a hub 537.

Frame Arrangements

FIGS. 6A and 6B illustrate schematic end and oblique views of a genericsolar collector module 600, and serve to illustrate the definitions ofsome terms useful in describing solar collector modules. The module 600includes a curved reflector 601 configured to concentrate incoming solarradiation 602 onto a receiving tube 603. The curved reflector 601 may bein the shape of a parabolic cylinder or other curved shape, and thereceiving tube 603 may be cylindrical. The curved reflector 601 may bemade of a material that is curved before mounting on the module 600, ormay be made of a flat planar material that adopts the curved shape whenplaced on the module 600. The module 600 has an axis of rotation 604,about which the module 600 is rotated to follow the apparent motion ofthe sun during the day. The receiving tube 603 and the axis of rotation604 define a central longitudinal plane 605 of the module 600, indicatedby dashed lines. The module 600 includes a three-dimensional structuralframe 606 to which the curved reflector 601 is attached. Thethree-dimensional structural frame comprises a plurality ofsubstantially rigid elongate structural members interconnected at nodes.The frame has a first end 607 and a second end 608. The nodes arearranged in generally longitudinal rows. For example, nodes 609 arearranged in a generally longitudinal row. Likewise nodes 610 arearranged in another generally longitudinal row. As will be appreciated,the nodes in a particular generally longitudinal row may be but need notbe perfectly collinear, and as is explained in more detail below, may beintentionally designed to be not collinear. The nodes also define anupper surface 611 and a lower surface 612. Each node in one surface isconnected by a strut to at least one node in the other surface.

Conventional frame designs, categorically designed around symmetric loadinput through a torque transfer assembly, are designed to distributeforces as evenly as possible among frame members. This makes membersizes more uniform (facilitating hub connections) and minimizes strengthrequirements for individual members. This is traditional practice for aspace-frame structure.

However a frame designed for transmitting torque to an adjacent framethrough a torque transfer connection near an edge or rim of the moduleis preferably configured in a way that quickly and directly transmitsforce inputs at one corner across the frame and out to the other corner.The edge or rim drive arrangement is described in pending U.S. patentapplication Ser. No. 12/416,536, previously incorporated by reference.For the purposes of this disclosure, a direct connection between modulesis one that connects the module frames directly through space, withoutpassing through an axle or shaft at the axis of rotation of the SCAincluding the modules. A direct connection may be made up of more thanone part.

FIGS. 7A and 7B illustrate oblique and top views of a frame 700 in whichcertain members are reinforced as compared with other members, to carryspecific loads, for example loads imparted by a rim or edge drive. Forthe purposes of this disclosure, for a first member to be reinforced ascompared with a second member means that the first member is configuredto safely carry a larger load than the second member. The reinforcementmay be accomplished by making the first member of a larger size, forexample a larger diameter, than the second member, by the first memberhaving thicker walls than the second member, or by other designdifferences.

The frame 700 includes a first end 701 and a second end 702 and an axisof rotation 703. Fittings 704 and 705 at the ends 701 and 702 aredisplaced from the axis of rotation 703, and are configured toparticipate in the transfer of torque between a module built upon theframe 700 and adjacent modules. Examples of fittings suitable for use asthe fittings 704 and 705 may be found in U.S. patent application Ser.No. 12/416,536, previously incorporated by reference. The frame 700 hasan upper surface 706 defined by a set of upper nodes and a lower surface707 defined by a set of lower nodes. Each node is connected by at leastone strut to at least one node in the opposite surface. In the frame700, the arrangement of the struts, upper diagonals, or both isasymmetrical about a central longitudinal plane 708. For example, thestruts 709 and 710 are reinforced as compared with their counterparts711 and 712 on the opposite side of the central longitudinal plane 708.In another example, diagonal member 713 in the upper surface of theframe 700 does not have a counterpart on the opposite side of thecentral longitudinal plane 708.

FIG. 7C shows another frame 714, including another example of frameasymmetry. In the frame 714, the lower surface is defined by nodes 715,arranged in one generally longitudinal row, and nodes 716, arranged inanother generally longitudinal row. The nodes 715 are connected by oneor more chord members 717. However the nodes 716 are not connected bychord members. Analysis has shown that the omission of chord membersbetween the nodes 716 may not have a significant effect on the strengthor stiffness the frame 714, especially when torque is transmittedbetween adjacent modules using an edge or rim drive arrangement.

In another example type of frame asymmetry, three nodes in a particulargenerally longitudinal row are not collinear. For example, node 718shown in FIG. 7C may be displaced inward and upward into the frame. Theframe members associated with node 718 may be accordingly shortened,resulting in material savings. FIG. 7D illustrates an end view of aframe having a traditional layout wherein the nodes in a particulargenerally longitudinal row are collinear, and FIGS. 7E and 7F illustrateend views of asymmetric frames.

In FIG. 7D, lower surface 719 is defined by nodes in two generallylongitudinal rows 720 and 721. In this traditional arrangement, thenodes in the rows 720 and 721 are collinear, so that in the orthogonalend view of FIG. 7D, the nodes appear superimposed. In FIG. 7E, a node722 has been displaced inward and upward within the frame. Struts 723and 724 and lower diagonal 725 may be accordingly shortened as comparedwith analogous members in the frame of FIG. 7D. The nodes in generallylongitudinal row 726, including the node 722, are not collinear, due tothe displacement of the node 722. In the example frame of FIG. 7E, thenode 722 is connected to other nodes in its generally longitudinal rowby at least one chord member 727.

The example frame of FIG. 7E is similar that of FIG. 7D, except that nochord member connects the node 722 to the other nodes in its generallylongitudinal row. This example illustrates that the various innovationsdescribed herein may be used in combination. FIG. 7F illustrates the useof a displaced node and an omitted framing member in combination.

FIGS. 7G and 7H illustrate end views of collector modules exhibitingextreme frame asymmetry. The primary benefit of an asymmetric structurefor a solar collector module frame is material efficiency. In the caseof a frame which is designed to be driven by the rim, one side may havemore support structure than the other. By leaving out unnecessarymembers on the non-drive side, or by changing the node configuration onthe drive side, the overall structure can meet design requirements withless material. FIG. 7H also illustrates that an axis of rotation 728 ofa frame need not be aligned with an axis of symmetry 729 of a reflector730.

Discrete Frame Members

In some prior solar collector module frames, the chord members extendsubstantially the entire length of the frame. In accordance with anotherembodiment of the present invention, the chord members in a module framedo not run the entire length of the frame, but the chords are broken upinto discrete units, each discrete unit joining only two nodes. Becausethe other kinds of framing members (diagonals, struts, and ribs) alsoextend only from one node to one other node, a frame in accordance withthis embodiment has the property that no frame member reaches more thantwo nodes. This arrangement may have advantages in the fabrication ofmembers to high precision. Transport and assembly of the framecomponents may also be simplified, as the members and subassemblies thatneed to be manipulated during assembly are smaller than in a framehaving full-length chords. For the purposes of this disclosure, a framemember “reaches” a node when the frame member or a frame member assemblyincorporating the frame member passes through or is connected to a hubat the node.

This embodiment may be especially useful in combination with hubs suchas the hub 314 shown in FIG. 3F. Because the hub does not need toaccommodate a chord member passing through it, the hub may be smaller,use less material, and require less elaborate tooling for itsfabrication. Having the chord members broken up into discrete segmentsalso facilitates the design and construction of asymmetric frames asillustrated in FIGS. 7E and 7F, because the nodes in any particulargenerally longitudinal row of nodes are not constrained by a single longchord to be collinear.

A module in accordance with this embodiment includes a curved reflectorand a three-dimensional structural frame to which the reflector ismounted. The frame is made of a plurality of interconnectedsubstantially-rigid frame members connected at nodes. The frame includesa set of upper nodes defining an upper surface of the frame, and a setof lower nodes defining a lower surface of the frame. A hub is presentat each node. Each frame member reaching a particular node is connectedto the respective hub at that particular node. No framing member,including the chord members, reaches more than two nodes. FIG. 8Aillustrates one node connection in a module frame in accordance withthis embodiment. In the node connection of FIG. 8A, a monolithic hub801, made of a single piece of material, receives a chord member 802, arib 803, a diagonal 804, and a strut 805. Each framing member terminatesat the hub 801. A second chord member 806 may also be connected to thehub 801.

This arrangement of discrete chord segments enables other designalternatives as well. For example, different chord members within aparticular generally longitudinal row of nodes may be made of differentsizes to accommodate different design load expectations. FIG. 8B showsan example solar collector module 807 in which three chord segments 808,809, and 810, all connecting nodes within the same generallylongitudinal row of nodes, are not all of the same cross sectional size.The three chord members may have different load requirements, such thatthe center chord member 809 need not withstand loads as high as thoseexperienced by the members 808 and 810. In that case, the center chordmember 809 may have a smaller cross section (for example a smallerdiameter), so that material and cost are saved, as compared with makingthe chord member 809 as large as the members 808 and 810. Other framegeometries may result in different chord sections being larger orsmaller.

In another example embodiment, one or more chord members may be omittedentirely. For example, chord member 811 (the center segment at the uppercenter of the frame), shown in FIG. 8B, may be omitted. In thisembodiment, with the chord member 811 omitted, two nodes 812 and 813 ina particular row of nodes are connected by a chord member 814, but adifferent set of nodes 813 and 815 in the same row are not connected bya chord member. In other embodiments, none of the nodes in a row ofnodes may be connected by chord members.

Full-Length Hubs

In accordance with another embodiment, the use of separate hubs may beavoided. FIG. 9A shows a portion of a three-dimensional structural frame900, suitable for use as a frame for a solar collector module, inaccordance with this embodiment. The frame 900 includes a set of uppernodes defining an upper surface (not shown). The upper nodes arepreferably arranged in generally longitudinal rows. The frame 900 alsoincludes a set of lower nodes defining a lower surface, also arranged ingenerally longitudinal rows. In FIG. 9, two nodes 901 and 902 are shown.Various structural members, including struts 903 and ribs 904, mayconnect at the nodes. Each node in the upper surface is connected to atleast one node in the lower surface by at least one of the struts 903.

A substantially rigid elongate chord member 905 extends to at least twonodes in a particular row of nodes, including the nodes 901 and 902.Similar chord members may be provided at other rows of nodes in theframe 900. The chord member 905 has a substantially constant crosssectional shape throughout its length, and may conveniently be made ofextruded aluminum, although other materials and fabrication processesmay be used. The chord member 905 includes connection features forconnecting the various other framing members. In the embodiment of FIG.9, the connection features are protruding tabs 906. The tabs 906 may be,for example, unitary fins, parallel fins, or may be tabs as illustratedin FIG. 9. A tab may preferably have two spaced apart outer sides and aspanning surface joining the outer sides. A tab may preferably have anaspect ratio between 0.25 and 4.0, and more preferably between 0.33 and3.0, and even more preferably between 0.5 and 2.0. FIG. 9B shows aportion of the frame of FIG. 9A in more detail, with several framemembers removed for purposes of illustration. Details of the tabconstruction are described above in conjunction with FIGS. 3A-3G.

While the embodiment of FIG. 9 may result in somewhat higher materialusage than using hubs connected by chord members, the embodiment of FIG.9 uses fewer parts, and may facilitate assembly of the frame 900. Thechord member 905 may be manufactured in the same way as extruded hubssuch as the hub 309 shown in FIG. 3C. For example, an extrusion havingthe cross section of the chord member 905 may be cut into short lengthsto form hubs, and into longer lengths to form chord members such as thechord member 905.

Attached Channel Hub

FIG. 10 illustrates another kind of connection at a node of a frame, inaccordance with another embodiment. In FIG. 10, a chord member 1001 runslongitudinally within a frame similar to those described above,extending to some or all of the nodes within one generally longitudinalrow of nodes in one surface of the frame. The chord member 1001 may havea constant cross sectional shape along its length, and is convenientlymade by extrusion of aluminum, although other materials and processesmay be used. The chord member 1001 is shown as generally round withintegrated flattened surfaces, but could be square, rectangular, ofanother polygonal shape, oval, elliptical, or of any other suitableshape. The chord member 1001 is preferably hollow, as shown. A pluralityof struts may connect the nodes, such that each node in the uppersurface is connected to at least one node in the lower surface by atleast one of the struts.

Affixed to an outside surface of the chord member 1001 are one or morechannels 1002. Each of the channels 1002 has a throat 1003 and twospaced apart sides 1004. Each of the channels 1002 is affixed at itsthroat 1003 to the chord member 1001, and is configured to receive atleast one strut member such as member 1007, for example between thesides 1004. The channels 1002 may be affixed to the chord member 1001 byany suitable means, for example by rivets, bolts, screws, adhesivebonding, welding, or an integral attachment feature such as a dovetailjoint. Multiple means may be used for affixing the channels 1002 to thechord member 1001. In the example of FIG. 10, the channels 1002 arepoised to be affixed using rivets 1005. The channels 1002 may also beconveniently made by extrusion, and may be shaped to conform to theoutside surface of the chord member 1001. Each of the channels 1002 ispreferably aligned toward other nodes to which members connecting to therespective channel 1002 also connect. The channels 1002 can extend alongthe chord member 1001 a distance sufficient to receive as many framemembers as connect at the node.

The node connection of FIG. 10 may be advantageous in that frame memberscan be received between the spaced-apart sides 1004 of the channels1002, without the need for frame member end connectors. For example,each channel may comprise one or more holes 1006 cooperativelypositioned with holes in the end of a member being received, so that theframe member and the channel can be connected by one or more pins,bolts, screws, or other fasteners.

Mirror Standoffs

FIG. 11A shows a portion of a solar collector module in accordance withother embodiments. A space frame made with straight members may have anupper surface whose members (chords, ribs, and diagonals) definesubstantially flat planes. The entire surface need not be flat, and maybe V-shaped to approximate the curved shape of a reflector to be held inplace by the frame. However, transition elements may still be requiredto accommodate the remaining shape differences and to support thereflector in its proper position. In some prior designs, purlins wereplaced on the upper surface of the frame, running the length of theupper surface. The purlins were selected to have a height that wouldspace the reflector, resting on the purlins, properly from the frameelements. Full length purlins require substantial material, and may addweight and cost to a solar collector module.

In the embodiment of FIG. 11, a solar collector module comprises acurved reflector 1101 and a three-dimensional supporting structuralframe to which the reflector is attached. For example, the frameincludes rim chord member 1102, struts 1103, and ribs 1104,interconnected by hubs 1105. Discrete standoffs 1106 are placed atspaced locations along at least the rim chord member 1102. The standoffs1106 are sized to support the reflector 1101 at the proper distance fromthe chords so that the reflector 1101 performs properly to concentrateincoming solar radiation onto a receiving tube (not shown in FIG. 11).When the chord member 1102 is parallel to the axis of the reflector 1101and the supporting frame, then the standoffs placed along the chordmember 1102 may be all of the same size. Standoffs of other sizes may beused at other chord members.

The standoffs 1106 are preferably made of aluminum formed principally byextrusion, although other materials and processes may be used. Thestandoffs 1106 may be affixed to the chord members such as the chordmember 1102 by any suitable means, including rivets, bolts, screws, orwelding. The reflector 1101 may be made, for example, of one or moresegments of aluminized glass, polished metal, or another suitablyreflective material. The reflector 1101 may be attached to the standoffs1106 by any suitable means, including bolts, screws, adhesives, oranother attachment means.

FIG. 11B shows a portion of a solar collector module in accordance withother embodiments, and illustrates standoffs 1107 of an alternativedesign. The standoffs 1107 are also preferably made by extrusion and cutto length, and are shaped to engage with chord member 1108. As isillustrated by standoff 1109, not all of the standoffs need be of thesame length, and additional machining operations may be performed on thestandoffs such as the standoff 1109, for example to span nodes. Thestandoffs 1107 and 1109 may be affixed to the chord member 1108 andattached to the reflector 1101 in ways similar to those discussed above.

The present invention has been described above in terms of presentlypreferred embodiments so that an understanding of the present inventioncan be conveyed. There are, however, many configurations for collectorsystems not specifically described herein but with which the presentinvention is applicable. The present invention should therefore not beseen as limited to the particular embodiments described herein, butrather, it should be understood that the present invention has wideapplicability with respect to collector systems generally. Allmodifications, variations, or equivalent arrangements andimplementations that are within the scope of the attached claims shouldtherefore be considered within the scope of the invention.

What is claimed is:
 1. A solar collector module, comprising: a reflectorconfigured to direct incoming solar radiation onto a receiver; and athree-dimensional structural frame to which the reflector is mounted,the three-dimensional structural frame comprising a plurality ofinterconnected substantially rigid elongate frame members, thethree-dimensional structural frame having two ends and furthercomprising at each end a fitting displaced from an axis of rotation ofthe solar collector module, each fitting configured to participate inthe transfer of torque between the solar collector module and anadjacent module via a direct connection between the solar collectormodule and the adjacent module at a location displaced from the axis ofrotation; wherein the three-dimensional structural frame is asymmetricalabout a central longitudinal plane.
 2. The solar collector module ofclaim 1, wherein certain frame members are reinforced as compared withother frame members in order to carry specific loads imparted by thedirect connection.
 3. The solar collector module of claim 1, wherein atleast one frame member on a first side of the central longitudinal planeis of a different size than a counterpart of the frame member on asecond side of the central longitudinal plane.
 4. The solar collectormodule of claim 1, wherein at least one frame member of a first side ofthe central longitudinal plane does not have a counterpart on a secondside of the central longitudinal plane.
 5. The solar collector module ofclaim 1, wherein the frame members are connected at nodes, and whereinthe three-dimensional structural frame comprises an upper surfacedefined by a set of upper nodes and a lower surface defined by a set oflower nodes, each lower node connected to at least one upper node by astrut, wherein at least one lower node on a first side of the centrallongitudinal plane is connected to another lower node on the first sideof the central longitudinal plane by a chord member running generallyparallel to the axis of rotation, and wherein at least one lower node ona second side of the central longitudinal plane is not connected to achord member.
 6. The solar collector of claim 1, wherein the framemembers are connected at nodes, and wherein the three-dimensionalstructural frame comprises an upper surface defined by a set of uppernodes and a lower surface defined by a set of lower nodes, and whereinthere are at least three lower nodes in a generally longitudinal row onone side of the central longitudinal plane, and wherein the three lowernodes are not collinear.
 7. The solar collector of claim 1, wherein thereflector is curved.
 8. A solar collector module, comprising: areflector configured to direct incoming solar radiation onto a receiver;a three-dimensional structural frame to which the reflector is mounted,the three-dimensional structural frame comprising a plurality ofinterconnected substantially rigid elongate frame members, wherein theframe members are connected at nodes, and wherein the three-dimensionalstructural frame comprises an upper surface defined by a set of uppernodes and a lower surface defined by a set of lower nodes, each lowernode connected to at least one upper node by a strut; and a monolithichub at each node to which each frame member reaching the respective nodeis connected; wherein no frame member reaches more than two nodes, andwherein the nodes are arranged in generally longitudinal rows, and thenodes in at least one row of lower nodes are not collinear.
 9. The solarcollector module of claim 8, wherein a set of two nodes in one row areconnected by a chord member of a first cross sectional size, and adifferent set of two nodes in the same row are connected by a chordmember of a second cross sectional size different from the first. 10.The solar collector module of claim 8, wherein a set of two nodes in onerow are connected by a chord member, and a different set of two nodes inthe same row are not connected by any chord member.
 11. A solarcollector module, comprising: a reflector configured to direct incomingsolar radiation onto a receiver; a three-dimensional structural frame towhich the reflector is mounted, the three-dimensional structural framecomprising a plurality of interconnected substantially rigid elongateframe members, wherein the frame members are connected at nodes, andwherein the three-dimensional structural frame comprises an uppersurface defined by a set of upper nodes and a lower surface defined by aset of lower nodes, each lower node connected to at least one upper nodeby a strut; and a monolithic hub at each node to which each frame memberreaching the respective node is connected; wherein no frame memberreaches more than two nodes; and wherein the nodes are arranged ingenerally longitudinal rows; and wherein, within at least one particularrow, no two nodes are connected by chord members.
 12. The solarcollector module of claim 11, wherein the reflector is curved.
 13. Thesolar collector module of claim 11, further comprising at least onesubstantially rigid elongate chord member in each row of the uppersurface, connecting at least two nodes in the respective row; and aplurality of discrete standoffs directly affixed to the chord members inthe upper surface, the standoffs at least partially supporting thereflector.
 14. A solar collector module, comprising: a reflectorconfigured to direct incoming solar radiation onto a receiver; athree-dimensional structural frame to which the reflector is mounted,the three-dimensional structural frame comprising a plurality ofinterconnected substantially rigid elongate frame members, wherein theframe members are connected at nodes, and wherein the three-dimensionalstructural frame comprises an upper surface defined by a set of uppernodes and a lower surface defined by a set of lower nodes, each lowernode connected to at least one upper node by a strut, and wherein thenodes are arranged in generally longitudinal rows; at least onesubstantially rigid elongate chord member in each row of the uppersurface, connecting at least two nodes in the respective row; and aplurality of discrete standoffs directly affixed to the chord members inthe upper surface, the standoffs at least partially supporting thereflector; and wherein the nodes in at least one row of lower nodes arenot collinear.
 15. The solar collector module of claim 14, wherein eachstandoff is configured to be made by a process comprising extrusion. 16.The solar collector module of claim 14, wherein the standoffs areaffixed to the respective chord members by means selected from the groupconsisting of bolts, screws, and welding.
 17. The solar collector moduleof claim 14, wherein the reflector is curved.